1. Field of the Invention
The invention relates generally to an optical disk device and a control method for the optical disk device, and is suitable for use in, for example, an optical disk device compatible with DVDs (Digital Versatile Discs).
2. Description of Related Art
Mark edge recording—a method of recording information on the edges of a recording mark—has been used as one of the methods for recording data on optical disks. This mark edge recording has an advantage over mark position recording—a method of recording information in the center of the recording mark—in high density recording.
However, as discussed in JP08-115521 A, mark edge recording has a drawback in that a recording mark, which is supposed to have a specified length when recorded, may have a different length, depending on the optical power during data recording. When the recording mark has a length different from its proper length, the positions of the edges of the recording mark are shifted, which inhibits correct reading of data recorded on the optical disk.
There are various probable causes of variations in optical power during recording, one of which is defocus during data recording. In practice, defocus reduces the optical power density during data recording and causes a shortage of power, thereby inhibiting formation of a recording mark of the specified length. Defocus also lowers the resolution during data recording as well as data reproduction and may cause data read errors.
JP08-115521 A discloses a solution for the above problems, which involves, in 1-7 modulation mark edge recording, measuring during trial writing the amplitude (A) of a reproduction signal during the period corresponding to a minimum-length recording mark and a space having the same length as the minimum-length recording mark and the amplitude (B) of the reproduction signal during the period corresponding to a maximum-length recording mark and a space having the same length as the maximum-length recording mark; and setting a focus offset that maximizes the resolution value calculated by A/B.
In the method disclosed in JP08-115521 A, it is necessary to measure the value of the resolution while changing the position (“focus position”) of the objective lens relative to the optical disk so that the resolution value at each position can be obtained, in order to find a position where the resolution becomes the maximum.
However, when the resolution value is measured at each focus position, because the variations in the amplitude of a reproduction signal are different during the period corresponding to a long recording mark and during the period corresponding to a short recording mark due to the spherical aberration of the objective lens, the focus position exhibiting the maximum resolution is not necessarily the position causing less data read errors.
The invention has been devised taking into consideration the above problems and so it is an object of the invention to provide an optical disk device capable of recording and/or reproducing information on optical disks with high reliability by setting an optimum focus position; and a control method for the optical disk device.